San Diego Zoo's panda cub Xiao Liwu

The San Diego Zoo's panda cub, Xiao Liwu, was eager to play with a plastic ball during his 18th exam this morning in San Diego, Dec. 18, 2012. REUTERS/San Diego Zoo/Handout

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Xiao Liwu, the giant panda cub at the San Diego Zoo, appeared to enjoy the attention and new toys he was given during his weekly exam, Dec. 27, 2012. (San Diego Zoo/Ken Bohn/HO)

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Xiao Liwu, the San Diego Zoo's panda cub, is pictured in his first visit to his outside habitat at the zoo in San Diego, California in this January 9, 2013 handout photo. REUTERS/Ken Bohn/San Diego Zoo/Handout

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Xiao Liwu is seen at the San Diego Zoo in this Dec. 4, 2012 file photo. (SAN DIEGO ZOO/Ken Bohn/HO)

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The San Diego Zoo's panda cub, Xiao Liwu, started the New Year with a clean bill of health from his veterinarians, Jan. 1, 2013 in San Diego. (San Diego Zoo/Ken Bohn)

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San Diego Zookeepers placed a small section of a tree branch and some fresh bamboo on the ground for the panda cub's 17th exam, Dec. 13, 2012. (San Diego Zoo/Ken Bohn/HO)

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Comfortably curled up in a keeper's lap, the giant panda cub Xiao Liwu received his weekly veterinary exam on November 13, 2012 at the San Diego Zoo in San Diego, California. (San Diego Zoo/Tammy Spatt/HO)